This means, that for my object that will instanciate my DataMapperExt class, $is_dmz will be set to false. Thus _load_languages() method will not be called. So I will have to call _load_languages manually in the constructor of my child class.
In my opinion, line 421 should be replaced by :
$is_dmz = $this instanceof DataMapper;
OR
$is_dmz = is_a($this, 'DataMapper');